The present invention relates to an ion source for a mass spectrometer and a method of ionising a sample. The preferred embodiment relates to a mass spectrometer and a method of mass spectrometry.
Atmospheric Pressure Ionization (“API”) ion sources are commonly used to ionize the liquid flow from HPLC or UPLC chromatography devices prior to analyzing the resulting gas phase ions via a mass spectrometer. Two techniques which are most commonly used comprise Electrospray Ionization (“ESI”) and Atmospheric Pressure Chemical Ionization (“APCI”). ESI is optimal for moderate to high polarity analytes and APCI is optimal for non-polar analytes. API ion sources that combine both of these techniques have been proposed and realized in designs that simultaneously combine ESI and APCI ionization using geometries that ensure that the electric fields generated by each technique are shielded and are independent of one another. These so called “multimode” ion sources have the advantage of being able to ionize analyte mixtures containing a wide range of polarities in a single chromatographic run without the need to switch between different ionization techniques. U.S. Pat. No. 7,034,291 discloses a ESI/APCI multimode ionization source comprising an ESI ion source and a downstream corona needle and U.S. Pat. No. 7,411,186 discloses a multimode ESI/APCI ion source. The known multimode ion sources suffer from the problem of being mechanically complex.
Other universal or multimode ionization sources have been proposed for interfacing liquid chromatography to mass spectrometry. One such example is a Surface Activated Chemical Ionization (“SACI”) ion source which directs a vapour stream from a heated nebuliser probe towards a broad area charged target plate which is situated close to the ion inlet aperture of the mass spectrometer and 15-20 mm away from the end of the nebuliser. The spray point of the SACI ion source is within the heated nebuliser probe so that the typical distance between the spray point of the SACI ion source and the target plate is 70 mm. This geometry with a relatively large distance between the sprayer and the target produces a divergent spray with a dispersed reflected flow at the target which generally results in lower sensitivities when compared to optimized ESI and APCI sources. U.S. Pat. No. 7,368,728 discloses a known Surface Activated Chemical Ionisation ion source.
It is also known to place a small target in the form of a bead at close proximity to the nebulised spray point in impactor nebulisers which are used in atomic absorption spectroscopy. An impactor nebuliser is, for example, disclosed in Anal. Chem. 1982, 54, 1411-1419. The known impactor nebuliser is not used to ionise a sample.
It is desired to provide an improved ion source for a mass spectrometer.